/**
 * Created by zby on 2018/4/8.
 */
public class Diedai {

    public static void main(String args[]) {
        int[] temp = new int[args.length];
        int sum;
        for (int i = 0; i < args.length; i++) {
            temp[i] = Integer.parseInt(args[i]);
        }
        sum = fact(temp[0], temp[1]);
        if (sum == 0) System.out.println("error");
        else System.out.println(sum);
    }

    public static int fact(int n, int m) {
        if (m == 1) return n;
        else if (m == 0 || m == n) return 1;
        else if (n < m || n == 0) return 0;
        else return fact(n - 1, m - 1) + fact(n - 1, m);

    }
}
